This invention relates to a process for the preparation of ethylene copolymers. More particularly, this invention relates to a process for the low-pressure production of high-molecular weight, low density copolymers of ethylene by the copolymerization of ethylene with propylene and/or butene-1 using mixed catalysts of titanium trichloride and a trialkyl-aluminum or dialkylaluminum hydride compound in the liquid phase.
Polyethylenes (both homopolymers and copolymers with .alpha.-olefins) having high molecular weights are required for various applications, e.g., in the manufacture of loom pickers and other highly stressed parts in the textile industry, in machinery construction, and in the chemical industry, e.g., for feeding screws, gear wheels, runners, valves and slides. Using the Ziegler polymerization process, it is possible to manufacture high-molecular weight types of polyethylene with RSV values of about 17 (measured in a 0.03% solution in decahydronaphthalene at 135.degree.C., Ostwald-type viscosimeter), which corresponds to a molecular weight of about 1 million, calculated in accordance with the solution viscosity ##EQU1## see Wesslau, Kunstsoffe 49, page 320 (1959). These products exhibit a density of 0.94.
High-molecular weight types of polyethylene, i.e., having RSV values as defined above of 1.5 or higher, with a lower density, i.e., of 0.930 or lower, only recently have been described. Although it is possible using the high-pressure method to produce low density polyethylene having densities of 0.915 - 0.935, these polymers have molecular weights of only up to about 50,000, calculated according to the solution viscosity.
Polyethylene types of low density are required for various fields of application, e.g., extrusion blow molding to obtain hollow articles and films, for coatings, for flame spraying, and for rotational sintering. The types of polyethylene produced according to the high-pressure method, with a density range of 0.915 - 0.935 g/cm.sup.3, have the disadvantage that their molecular weights are too low for many applications. In accordance with the low-pressure method, it is difficult or even impossible to set low densities. Thus, according to the low-pressure polymerization processes of Ziegler and Phillips, polyethylenes are obtained having a density of 0.94 - 0.965 g./cm.sup.3.
By copolymerization with other olefins, such as propene and butene-1, it is also possible to produce copolymers having a density of 0.930 - 0.950 according to the low-pressure method. This is accomplished, e.g., pursuant to the teachings of British Patent No. 944,371, by polymerization in inert diluents such as butane, pentane, hexane, cyclohexane, isopropylcylohexane, benezene or mixtures of hydrocarbons with the aid of mixed catalysts of halogen-orthotitanic acid esters and organo-metallic compounds of aluminum, in the presence of 0.05 - 20% by volume of propylene and/or butene-1, based on the ethylene. However, in this process, a considerable excess of propene or butene- 1 is required in order to obtain copolymers having a butene-1 content of 2-3%. For example, at a polymerization pressure of 3 atmospheres gauge, using hexane as the diluent, 8% of butene-1 is required in the mixture of monomers in order to obtain a copolymer with 2% of butene-1. When polymerizing at 12 atmospheres gauge in order to obtain a higher polymerization velocity, the addition of 35% of butene-1is required to produce a copolymer having 2-3% of butene-1. This copolymerization, furthermore, is successful only with the use of high-percentage 94-99% purity butene-1. With the utilization of 50% strength butene-1 at 12 atmospheres gauge in hexane as the diluent, it is impossible to manufacture a copolymer with 2-3% of butene-1. Besides, this polyethylene obtained with 50% strength-butene-1 is, with 1.0 - 1.5% butene-1, so fine-grained that it cannot be separated by means of decantation. With the polymerization pressures of 20 atmospheres gauge and thereabove, nowadays customary, it is impossible even when using 95-99% strength butene-1 to obtain a copolymer with 2% of butene-1. Similar difficulties are also encountered in the production of ethylene-butene-propylene terpolymers and ethylene-propylene copolymers.
British Pat. No. 932,658 describes copolymers of ethylene with 2-olefins. In the examples, butene-2 is primarily empolyed. The mixed catalysts consist of organometallic compounds of metals of Group Ia, II, or IIIa and transition metal compounds, e.g., Ti-, V-, or Cr-halides, V- or Cr-acetylacetonates or V-halo-alcoholates. These catalysts are preferably present in colloidal distribution, in finely divided form, or in a complete solution. Crystalline copolymers and completely amorphous copolymers are produced but the yields are extremely poor. Such a process is uninteresting from a technical viewpoint, since the yields, in part, are even lower than the amounts of catalyst used, e.g., in Example 13 which uses a TiCl.sub.3 and Al(C.sub.2 H.sub.5).sub.3 catalyst. In the remaining examples, vanadium catalysts are used.
The process described in British Pat. No. 967,788 likewise results in very poor yields. The same catalysts are used as in British Pat. No. 932,658, and co- and/or terpolymers or ethylene with 1-olefins and 2-olefins are produced.
According to the processes disclosed in German Patent Application Nos. P 22 06 429.5 and P 22 14 271.8, corresponding to copending, commonly assigned U.S. Pat. application Ser. No. 332,035, filed Feb. 12, 1973, not pertaining to the state of the art, some of the aforementioned difficulties can be overcome if polymerization is carried out in a C.sub.4 or C.sub.3 -C.sub.4 solvent containing propylene and/or butene-1 monomers and at least 10 molar percent of butene-2. The catalysts utilized in these processes are mixed catalysts containing halogen orthotitanic acid esters and/or titanium trichloride and chlorine-containing organoaluminum compounds. According to these patent applications, activation with trialkyl aluminum compounds or dialkyl aluminum hydrides is unsuitable, since this causes the formation of mixtures of the individual homopolymers in addition to co- and terpolymers.
U.S. Pat. No. 2,956,989 teaches that a butene-2-containing solvent is entirely unsuitable for the production of copolymers of ethylene with a catalyst of titanium trichloride and alkyl aluminum compounds, since the butene-2 is polymerized to polybutene-2. Further work done by R. O. Symcox and reported in J. Polymer Sci., Part B, 2 (10):947-949 (1964) established that the product is not polybutene-2, but polybutene-1 since the butene-2 is isomerized to butene-1 under the effect of Ziegler-Natta catalysts, and this butene-1 is then polymerized to polybutene-1. The research done by Symcox is confirmed by further publications, e.g., T. Otsu in J. Polymer Sci., Part A 4 (6):1579-1593 (1966); Masao Iwamoto and Sadao Yuguchi (Bull. Chem. Soc. Japan 40:159-162 (1967); German Unexamined Laid-Open Application DOS No. 1,545,042; and French Patent No. 1,415,239. According to these numerous publications, a butene-2-containing solvent is completely infeasible for the production of ethylene copolymers with Ziegler-Natta catalysts, since in addition to the ethylene copolymers also larger amounts of polybutene-1 are bound to be formed.